Hot Corrosion Behavior and Mechanism of Ru-Containing Ni-Based Single-Crystal Superalloy Exposed to Molten Na2SO4 at 950 °C

In this study, the hot corrosion behavior of a low-Cr, Ru-containing nickel-based single-crystal (SX) superalloy under exposure to Na2SO4 corrosive environments at 950 °C for up to 200 h was systematically investigated. The hot corrosion kinetics of the alloy exhibit a typical two-stage characteristic, consisting of an incubation stage and an acceleration stage. Notably, the mass gain rate is reduced after 100 h of hot corrosion. During the initial incubation stage, the molten sulfate reduces the interfacial oxygen partial pressure, thereby facilitating the formation of discontinuous reticular Al2O3/Cr2O3 and metastable MoS2. As corrosion progresses, the MoS2 in the upper region of the oxide layer is consumed, while a relatively continuous MoS2 layer is formed as an internal sulfide layer. In addition, as MoS2 evolves and sulfur is continuously supplied from the deposited sulfate, abundant sulfur reacts with Ru to form thermodynamically stable RuS2. This indicates that Ru participates in high-temperature sulfidation reactions and accompanies the transformation of corrosion products from the metastable MoS2 phase to the thermodynamically stable RuS2 phase. The continued mass gain with a reduced rate beyond 100 h of hot corrosion suggests that internal sulfides may influence the sulfur diffusion kinetics, although the inward sulfur penetration remains ongoing.